Dwelling on stressful events can weaken the immune system and make people more
susceptible to illness, study finds.

Thinking about negative events can have a detrimental effect on your health as it increases levels of inflammation in the body, researchers found in a study of 34 health young women.

This inflammation, associated with the body’s response to trauma and infection, has been linked with a number of disorders and conditions.

During the study each woman was asked to give a speech about her candidacy for a job to two interviewers in white laboratory coats, who listened with "stone-faced" expressions.

Half of the group were then asked to contemplate their performance in the public speaking task, while the other half were asked to think about neutral images and activities, such as sailing ships or shopping for groceries.

The scientists then took blood samples. They found that when participants were asked to ruminate on the incident their levels of C-reactive protein, which is a marker of tissue inflammation, rose.

C-reactive protein is primarily produced by the liver as part of the immune system's initial inflammatory response. It rises in response to traumas, injuries or infections in the body.

It is widely used as a clinical marker to determine if a patient has an infection, but also if he or she may be at risk for disease later in life.

Dr Peggy Zoccola, the lead author of the study and an assistant professor at the Ohio University in the United States, said: "More and more, chronic inflammation is being associated with various disorders and conditions.

"The immune system plays an important role in various cardiovascular disorders such as heart disease, as well as cancer, dementia and autoimmune diseases."

The study is the first to directly measure this effect in the body.

Dr Zoccola said: "Much of the past work has looked at this in non-experimental designs.

"Researchers have asked people to report their tendency to ruminate, and then looked to see if it connected to physiological issues. It's been correlation for the most part."

The levels of C-reactive protein were shown to be significantly higher in the subjects who were asked to dwell on the speech.

For these participants, the levels of the inflammatory marker continued to rise for at least one hour after the speech.

During the same time period, the marker returned to starting levels in the subjects who had been asked to focus on other thoughts.

Now Dr Zoccola is working with Fabian Benencia in Ohio University's Heritage College of Osteopathic Medicine and Lauren Mente, a registered nurse and graduate student in the School of Nursing, to investigate the effect of rumination on additional inflammation markers.

She also hopes to study the issue in other populations, such as older adults, who might be vulnerable to rumination and health problems.

She was due to present the findings at the annual meeting of the American Psychosomatic Society in Miami.